fix(clickhouse): cut idle CPU from tiny parts and oversized merge pools

Observability writers flushed every few seconds with MinBatchSize unset,
creating constant small parts and merge load. Enable MinBatchSize with
MaxFlushWait, batch access logs more aggressively, and shrink ClickHouse
background pools for 3c hosts.

internal/apps/openflare/chwriter/writer.go

@@ -20,13 +20,19 @@ import (
 )
 
 const (
+	// Observability traffic is sparse (heartbeat ~10s/node). Prefer larger batches to
+	// cut ClickHouse parts/merges; MaxFlushWait bounds visibility lag for single-node labs.
 	observabilityQueueSize    = 5_000
 	observabilityMaxBatchSize = 500
-	observabilityFlushEvery   = 5 * time.Second
+	observabilityMinBatchSize = 20
+	observabilityFlushEvery   = 10 * time.Second
+	observabilityMaxFlushWait = 30 * time.Second
 
 	nodeAccessLogQueueSize    = 10_000
 	nodeAccessLogMaxBatchSize = 1_000
-	nodeAccessLogFlushEvery   = time.Second
+	nodeAccessLogMinBatchSize = 50
+	nodeAccessLogFlushEvery   = 2 * time.Second
+	nodeAccessLogMaxFlushWait = 5 * time.Second
 )
 
 var (
@@ -182,7 +188,9 @@ func mustNewObservabilityWriter[T any](name string, flush batchwriter.FlushFunc[
 		Name:          name,
 		QueueSize:     observabilityQueueSize,
 		MaxBatchSize:  observabilityMaxBatchSize,
+		MinBatchSize:  observabilityMinBatchSize,
 		FlushInterval: observabilityFlushEvery,
+		MaxFlushWait:  observabilityMaxFlushWait,
 	}
 	writer, err := batchwriter.New(
 		cfg,
@@ -203,7 +211,9 @@ func mustNewNodeAccessLogWriter() *batchwriter.Writer[analyticsmodel.NodeAccessL
 		Name:          "node_access_logs",
 		QueueSize:     nodeAccessLogQueueSize,
 		MaxBatchSize:  nodeAccessLogMaxBatchSize,
+		MinBatchSize:  nodeAccessLogMinBatchSize,
 		FlushInterval: nodeAccessLogFlushEvery,
+		MaxFlushWait:  nodeAccessLogMaxFlushWait,
 	}
 	writer, err := batchwriter.New[analyticsmodel.NodeAccessLog](cfg, analyticsrepo.BatchInsertNodeAccessLogs,
 		batchwriter.WithDropHandler[analyticsmodel.NodeAccessLog](func(item analyticsmodel.NodeAccessLog) {